Method and Apparatus for Receiving a Global Positioning System Signal Using a Cellular Acquisition Signal

ABSTRACT

Method and apparatus for a GPS device that uses at least one cellular acquisition signal is described. More particularly, a GPS device is configured to receive at least one cellular acquisition signal for obtaining benefits associated with AGPS with only a small subset of AGPS circuitry to interact with a cell phone network. This facilitates use of GPS devices without subscription to a cell phone service provider, thus avoiding cellular subscription fees.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of co-pending U.S. patent applicationSer. No. 09/993,335, filed Nov. 6, 2001, which is a continuation-in-partof U.S. patent application Ser. No. 09/884,874, filed Jun. 19, 2001, nowU.S. Pat. No. 6,560,534, which is a continuation-in-part of U.S. patentapplication Ser. No. 09/875,809, filed Jun. 6, 2001, now U.S. Pat. No.6,542,820. Each of the aforementioned related patent applications isherein incorporated by reference. This application contains subjectmatter that is related to co-pending U.S. patent application Ser. No.09/715,860, filed Nov. 17, 2000, now U.S. Pat. No. 6,417,801, which isherein incorporated by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to mobile wireless devices for mobilelocation systems, and more particularly to Global Positioning Satellite(GPS) receivers with improved indoor penetration for personal locationsystems.

2. Description of the Related Art

With the advent of GPS, there has been a growing demand for mobiledevices that may be used to provide a person's or an object's location.Devices built using conventional GPS receivers have been developed by anumber of companies. However, these devices have significantlimitations, one of which is indoor penetration.

To address the above limitation of conventional GPS receivers, acombination of mobile GPS receivers and cellular infrastructurecommunicating via wireless links has evolved. This combination oftechnologies, known as Assisted GPS (AGPS), combines a GPS receiver witha cellular handset. The cellular handset provides a two-way link forcommunicating positioning data (“aiding data”).

In particular, performance of a conventional GPS mobile device in indoorenvironments may be limited by ability of the GPS mobile device todecode a navigation data stream broadcast by each of a plurality ofsatellites. Among other components, each navigation data stream containsa satellite trajectory model having parameters describing a respectivesatellite's orbit and dock variation as a function of time. Thesatellite trajectory model in the navigation data stream is sometimesreferred to as “broadcast ephemeris.” GPS mobile devices traditionallyreceive and decode the navigation data stream to extract the broadcastephemeris, which is needed to compute position. However, asignal-to-noise ratio in indoor environments is often insufficient fornavigation data bit decoding of the broadcast ephemeris. Thus, anothermeans of ascertaining satellite orbit and clocks variations was needed.

In AGPS systems, the satellite orbit and dock variation, or informationderived from these components, is provided to the GPS mobile device viaa two-way cellular link. A two-way cellular link is used to request andreceive information on such satellites, and the AGPS service isconventionally available only to authorized subscribers to a cellularnetwork.

While AGPS offers improvements in indoor penetration, addition of acellular handset and a subscription to a wireless provider adds to thecost and power consumption of a GPS receiver. Cellular handsets containcomplex and costly components. For example, the cost of adding a cellphone alone to a GPS receiver may be prohibitive for GPS applicationswhere a phone would otherwise be an unnecessary addition, let alone theaddition of a subscription fee of a cellular provider. Moreover,cellular transmission consumes power.

Therefore, it would be desirable to provide a GPS mobile device that iscomparable in cost to conventional GPS handheld devices but with theindoor penetration benefits associated with AGPS handsets.

SUMMARY OF THE INVENTION

The present invention provides apparatus and method for obtainingbenefits associated with AGPS without requiring complete integration ofa GPS device with a cellular handset. Furthermore, the present inventionfacilitates a GPS handheld or mobile device configured to operatewithout subscription to a cell phone service provider, and thuseliminates fees for such subscription. An aspect of the presentinvention is a GPS handheld device that comprises a cellular acquisitionsignal receiver or front end. It will be appreciated that circuitryrequired to receive an acquisition signal comprises only a portion of acomplete cellular handset. Particularly, a transmitter portion forcommunicating with a base station of a cellular network is not includedin the GPS handheld device. Furthermore, digital signal processor andapplication processor(s) configured for modulating, demodulating, voiceprocessing, call protocols, subscriber identification and the like areabsent in the GPS handheld device. The cellular acquisition signalreceiver allows the GPS handheld device to have an accurate time of dayand/or frequency reference, thus assisting in GPS signal acquisition andGPS position computation.

An aspect of the present invention is a method for receiving a GPSsignal. More particularly, a frequency correction burst is obtained froma cellular network. A frequency offset responsive to the frequencycorrection burst is determined, and a window of frequency searchresponsive to the frequency offset is determined for receiving the GPSsignal. This may be done without having to transmit a cellular signal tothe cellular network, and this may be done without having to have asubscription to the cellular network.

Another aspect of the present invention is a method for receiving a GPSsignal to a GPS handheld device. More particularly, a timesynchronization burst is obtained from a cellular network. A timingoffset responsive to the time synchronization burst is determined, and atime of day responsive to the timing offset is determined for receivingthe GPS signal. This may be done without having to transmit a cellularsignal to the cellular network, and this may be done without having tohave a subscription to the cellular network.

Another aspect of the present invention is a method for determiningposition of a GPS handheld device in proximity to a cellular basestation of a cellular network. More particularly, at least one oflocation information and identification information is obtained from thecellular base station, and a position estimate of the GPS handhelddevice responsive to the at least one of location information andidentification information is determined. This may be done withouthaving to transmit a cellular signal to the cellular network, and thismay be done without having to have fee-based access to the cellularnetwork.

Another aspect of the present invention is GPS mobile device. Moreparticularly, the GPS mobile device comprises at least one antenna. Theat least one antenna is coupled to a cellular acquisition signal frontend couple to receive a cellular acquisition signal. A GPS signal frontend is coupled to the at least one antenna to receive a GPS signal. AGPS/cellular processor is coupled to the GPS front end and to thecellular acquisition front end. The GPS/cellular processor is configuredwith a GPS baseband processor in communication with the GPS front endand a cellular acquisition signal baseband processor in communicationwith the cellular acquisition signal front end. A reference oscillatoris coupled to the GPS/cellular processor. A general purpose processor iscoupled to the cellular acquisition signal baseband processor and to theGPS baseband processor, and memory is coupled to the general purposeprocessor.

BRIEF DESCRIPTION OF THE DRAWINGS

It is to be noted, however, that the appended drawings illustrate onlytypical embodiments of this invention and are therefore not to beconsidered limiting of its scope, for the invention may admit to otherequally effective embodiments.

FIG. 1 is a network diagram of an exemplary portion of an embodiment ofa GPS and cellular network in accordance with one or more aspects of thepresent invention.

FIG. 2 is a network diagram of an exemplary portion of an embodiment ofa GPS and computer network in accordance with one or more aspects of thepresent invention.

FIG. 3 is a flow diagram of an exemplary portion of an embodiment of amobile or handset GPS unit receiving cellular acquisition signals inaccordance with one or more aspects of the present invention.

FIG. 4 is a chip-level block diagram of an exemplary portion of anembodiment of GPS unit in accordance with one or more aspects of thepresent invention.

FIG. 5 is a signal detection diagram of an exemplary embodiment of afrequency and delay window in accordance with one or more aspects of thepresent invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention relates to implementing a GPS system using a GPShandheld configured for receiving acquisition signals from a cellularprovider without requiring integration of the GPS receiver with atwo-way capable cellular handset. The present invention provides one ormore benefits conventionally associated AGPS while eliminating therequirement for obtaining aiding data from a cellular network.

An aspect of the present invention is a method for configuring the GPSdevice to store satellite trajectory data that replaces assistance dataprovided in AGPS systems. In AGPS systems, receiving assistance datafrom a cellular network requires a two-way capable cellular handset torequest and receive such assistance data. Moreover, such services arecommonly available only to paying subscribers to the cellular network.In particular, in one aspect of the present invention, satellitetrajectory models are stored in memory in a GPS mobile device. Thesource of the satellite trajectory models stored in memory may bebroadcast ephemeris, received and decoded when a GPS mobile device isoutside in at least a medium signal strength signal environment in whichnavigation data decoding is feasible. Alternatively, satellitetrajectory models may be supplied to the GPS mobile device through acomputer network connection. If the satellite trajectory models comprisebroadcast ephemeris, the satellite trajectory models may be valid forbetween approximately two and six hours. In another aspect of theinvention, long-term satellite trajectory models are used, which may bevalid for days. Once the satellite trajectory models are obtained andare in memory, the GPS mobile device may function indoors for the periodof validity of the long-term satellite trajectory models. Thiseliminates the need to more frequently obtained assistance data as withconventional AGPS.

Another aspect of the present invention is a method for determining timeoffset of a GPS mobile device. More particularly, a cellular acquisitionsignal may comprise a time synchronization signal that is received bythe GPS mobile device, enabling the GPS mobile device to establish atime of day for applying satellite trajectory data. In addition, thetime synchronization signal, if sufficiently precise, may be used toestablish a delay search window, decreasing the search time required forGPS signal acquisition. In addition, the time synchronization signal maybe used to align coherent averaging intervals with GPS signal data bitsto improve signal to noise ratio.

Another aspect of the present invention is a method for determining timeoffset of a GPS mobile device. More particularly, a cellular acquisitionsignal can comprise a time synchronization signal that is received atthe GPS mobile device, enabling the GPS mobile device's receiver toestablish a time of day for applying satellite trajectory data. Inaddition, the time synchronization signal, if sufficiently precise, maybe used to establish a delay search window, decreasing search timerequired for GPS signal acquisition. In addition, the timesynchronization signal may be used to align coherent averaging intervalswith GPS signal data bits to improve signal-to-noise ratio.

Another aspect of the present invention is a GPS mobile devicecomprising: one or more antennas configured to receive cellularacquisition signals and GPS satellite signals; radio frequency (RF)front end circuitry for the GPS signals; RF front end circuitry for thecellular acquisition signals; a cellular acquisition signal basebandprocessor; a GPS signal baseband processor; a time keeping countercommon to the baseband processors; a reference oscillator coupled to thetime keeping counter, baseband processors and front end circuitry; aprocessor coupled to the baseband processors; and memory coupled to theprocessor. In some embodiments, the GPS mobile device may additionallycomprise a computer network-docking interface or a data modem or both.

In the following description, numerous specific details are set forth toprovide a more thorough understanding of the present invention. However,it will be apparent to one of skill in the art that the presentinvention may be practiced without one or more of these specificdetails. In other instances, well-known features have not been describedin order to avoid obscuring the present invention.

Referring to FIG. 1, there is shown a network diagram of an exemplaryportion of an embodiment of a GPS and cellular network in accordancewith one or more aspects of the present invention. Satelliteconstellation 11 comprises a plurality of satellites. For purposes ofillustration four satellites, namely, satellites 11-1, 11-2, 11-3 and11-4, are shown, though fewer or more satellites may be used. GPS device10 is configured to receive one or more satellite signals 12 fromsatellite broadcast. GPS device 10 is configured to receive satellitebroadcast signals 12 as a form of one-way communication. GPS device 10is configured to receive one or more cellular broadcast signals 14 fromcellular base station 13. GPS device 10 is configured to receivecellular broadcast signals 14 as a form of one-way communication. GPSdevice 10 may be configured to operate to receive satellite informationfrom satellite broadcast signals 12 or a computer network connection, asdescribed below in more detail, or both. Furthermore, especially whenoperating in indoor or other satellite signal-harsh environments, one ormore cellular acquisition signals 14 broadcast from communication tower13 is utilized by GPS device 10.

Referring to FIG. 2, there is shown a network diagram of exemplaryportion of an embodiment of a GPS and a computer network for obtainingsatellite information, such as one or more satellite trajectory models,in accordance with one or more aspects of the present invention. GPSmobile device 10 may be put in communication with computer 22. Computer22 may be put in communication with computer network 21, which may forma portion of an intranet or the Internet. Network 21 may be put incommunication with server 23. Server 23 comprises or has access todatabase 27. Database 27 comprises one or more satellite trajectorymodels 39, such as for respective satellites 11 of FIG. 1. Accordingly,server 23 may be in communication with one or more GPS receiver stations27-1, 27-2, and 27-3 via network 21 for receiving broadcast ephemeriscomprising satellite trajectory models 39. GPS mobile device 10 may haveone or more satellite trajectory models 39 downloaded to it from server23.

Alternatively, server 23 may be put in communication with publiclyswitched telephone network (PSTN) 25 via network 21. PSTN 25 may be putin communication with telephone 26, which may be put in communicationwith GPS mobile device 10. In this embodiment, a phone number, such as atoll free number, may be dialed in order to download one or moretrajectory models to GPS mobile device 10.

Connection between mobile device 10 and server 23 may be established torefresh satellite trajectory models 39. At other times, this connectionmay be absent. For example, in field conditions lacking computer network21 connectivity, GPS handheld 10 may obtain satellite information fromone or more satellite signals 12 shown in FIG. 1. Such information istypically valid for approximately two to six hours from time ofbroadcast. Before the validity period ends, a GPS receiver should attainanother valid broadcast of ephemeris information to continue to operate.

In another embodiment, satellite tracking data from GPS referencestations 27-1, 27-2, and 27-3 is used in server 23 to create long-termsatellite trajectory models 39, which may be valid for periods of up toapproximately one week. Orbit models and associated long-term orbittrajectory data are described in more detail in co-pending and relatedapplications entitled “LONG TERM EPHEMERIS” to James W. LaMance, CharlesAbraham and Frank van Diggelen, application Ser. No. 09/884,874, filedJun. 19, 2001, and “METHOD AND APPARATUS FOR GENERATING AND DISTRIBUTINGSATELLITE TRACKING” to James W. LaMance, Charles Abraham and Frank vanDiggelen, application Ser. No. 09/875,809, filed Jun. 6, 2001. In oneaspect of the present invention, long-term orbit trajectory models areused in GPS mobile device 10 to extend the period of validity ofsatellite trajectory models 39 provided by server 23. This increases theinterval over which GPS mobile device 10 may be used in conditionswherein computer network 21 is not readily accessible.

Referring to FIG. 3, there is shown a flow diagram of an exemplaryportion of an embodiment of a mobile or handset GPS unit receivingcellular acquisition signals in accordance with one or more aspects ofthe present invention. Cellular base station 13 broadcasts severalcellular acquisition signals including frequency correction signal 31,time synchronization signal 32, timing message, such as a frame number,signal 33, and cell identification number signal 34A. Notably, timemessage signal 33 may be a separate signal or may be a time message 33provided with time synchronization signal 32. In some embodiments ofcellular base station 13, an additional signal, namely cell locationsignal 34B, is provided. It should be understood that one or more ofthese broadcast elements 31, 32, 33, 34A and 34B may be present orabsent in particular cellular network implementations. Furthermore, insome cellular networks one or more of broadcast elements 31, 32, 33, 34Aand 34B may be combined into various combinations of composite signals.In accordance with one or more aspects of the present invention, one ormore of these signals 31, 32, 33, 34A and 34B may be utilized, whetherindividually, jointly, or in various combinations.

Conventionally, cellular acquisition signals are provided to enable, atleast in part, a cellular handset to synchronize to a cellular basestation, as a first step in establishing communication with a cellularnetwork. In particular, in the first phase of establishingcommunication, the cellular handset monitors specific frequencies forthe acquisition signals. In accordance with one or more aspects of thepresent invention one or more cellular acquisition signal is received,but a GPS mobile device does not continue with the subsequent stepsneeded to establish two-way communication with the cellular network. Inparticular, in accordance with one or more aspects of the presentinvention, a GPS mobile device does not transmit any data or message orboth to the cellular network. Furthermore, a GPS mobile device mayreceive one or more acquisition signals without a cellular networksubscription.

Referring to FIG. 4, there is shown a block diagram of an exemplaryportion of an embodiment of a mobile or handheld GPS 10 in accordancewith one or more aspects of the present invention. With continuingreference to FIG. 4 and additional reference to FIG. 3, extra circuitryis added to a conventional GPS receiver to allow one or more cellularacquisition signals 31, 32, 33, 34A and 34B, collectively and singlycellular acquisition signals 102, to be received. This includes a secondradio frequency (RF) tuner, namely cellular acquisition front end 131coupled to an additional antenna, namely antenna 111. One or morecellular acquisition signals 102 are received by antenna 111 andprovided from cellular acquisition front end 131 to cellular acquisitionsignal baseband 136. Cellular acquisition signal baseband 136 is used tolock and decode one or more cellular acquisition signals 102, forexample, using conventional digital processing well known in the designof cell phones. For cost considerations, cellular acquisition front end131 may be integrated into a conventional GPS front end 132, which iscoupled to GPS antenna 133, on a single RF semiconductor integratedcircuit 130. Moreover, to save cost, cellular acquisition signalbaseband 136 may be integrated with a conventional GPS baseband 137 on asingle digital signal processing semiconductor integrated circuit toprovide a GPS/cellular processor 135. Examples of such integratedcircuits include, but are not limited to, a digital signal processor(DSP). However, more than one integrated circuit may be used, forexample, a DSP and an application specific integrated circuit (ASIC),and a DSP and an FPGA. Accordingly, by providing integrated circuits 130and 135, only a marginal increment in cost is added to a conventionalGPS. Furthermore, other technologies such as radio frequency CMOS(complimentary-metal-oxide-semiconductor) allow integration of basebandprocessor functions and front end functions into a single ASIC. Includedin this marginal incremental cost are additional filters and an extraantenna, described in more detail with respect to FIG. 4. Moreover,because cellular acquisition signals 102 are relatively high in power, asimple antenna may be used in order to control costs even further.Alternatively, a single antenna 100 capable of receiving both GPS andcellular signals may be employed.

The nature of a frequency correction signal 31 varies depending on thecellular network. In CDMA systems, frequency correction signal 31 maycomprise a pilot channel. The pilot channel is a common channel that isbroadcast over a cell coverage area. Conventionally, the pilot channeluses a repeating pseudonoise (PN) sequence of 2¹⁵ chips. Multiple basestations transmit the same PN code but at different timing offsets toavoid mutual interference. To detect the pilot channel, GPS mobiledevice 10 may scan a range of PN code offsets until energy is detected,indicating a cellular base station transmitter, using a cellularacquisition front end 131 of FIG. 4. By phase or frequency locking to adetected pilot signal, GPS mobile device 10 may measure a frequencyoffset 35A related to error in a reference oscillator of GPS mobiledevice 10, such as reference oscillator 138 of FIG. 4. Alternatively,GPS mobile device 10 may make an open loop measurement of frequencyerror of a pilot signal to determine frequency offset 35A.

In GSM systems, frequency correction signal 31 is a frequency correctionburst transmitted periodically by a base station on one of severalfrequency channels assigned to the base station. The frequencycorrection burst signal 31 is an unmodulated tone transmitted at aspecific offset from a carrier frequency of the channel. GPS mobiledevice 10 may make an open loop measurement of frequency error from thefrequency correction burst to determine frequency offset 35A.

As one or more of broadcast signals 31, 32, 33, 34A and 34B arewell-known, for example in General System Mobile (GSM) systems and CodeDomain Multiple Access (CDMA) systems, among other known cellularsystems, unnecessary details regarding such signals are not repeatedhere for purposes of clarity.

In an alternative embodiment, optional reference oscillator steeringcircuit 142 of FIG. 4 is used to provide reference oscillator steering35B. In this embodiment, frequency correction signal 31 is used inconnection with steering circuit 142 (shown in FIG. 4) to maintainreference oscillator 138 (shown in FIG. 4) within its nominal operatingfrequency range. Steering circuit 142 may comprise a digital-to-analogconverter connected to a voltage control input of an oscillator 138(shown in FIG. 4). Frequency steering a reference oscillator based on afrequency correction signal is well known in cellular handsets and isgenerally a requirement to ensure that the handset transmitter isprecisely maintained at an assigned transmission frequency. However, asGPS mobile device 10 does not comprise a cellular signal transmitter,there is no requirement to maintain a transmission frequency.Furthermore, GPS devices, in contrast to AGPS devices, tend to operateoff a non-steered reference oscillator, such as an oscillator withoutvoltage control. Thus, a non-steered reference oscillator 138 (shown inFIG. 4) may be used along with frequency correction signal 31 toprovide, or compute such as with cellular acquisition baseband processor136 (shown in FIG. 4), frequency offset 35A. Frequency offset 35A isprovided to frequency and delay search window 41.

An example of a frequency and delay search window 45 for a particularGPS satellite signal 11 is shown in FIG. 5. As will be understood tothose familiar with the art, frequency and delay window 45 comprises atwo dimensional space of uncertain frequency on frequency window 501axis and uncertain code delay on delay window 502 axis. To acquire a GPSsignal, a GPS receiver 10 (shown in FIG. 4) searches frequency and delaysearch windows 501 and 502, respectively for a GPS signal. An exemplaryGPS signal response 503 is shown in FIG. 5. GPS receiver 10 (shown inFIG. 4) detects signal response 503 by scanning with one or more searchbins 504. If frequency and/or delay uncertainty is large, this searchcan be time consuming. This is especially true in indoor environmentswhere, in order to obtain needed signal-to-noise ratio enhancements, GPSreceiver 10 (shown in FIG. 4) dwells for periods of several secondsaccumulating signal power before advancing search bin 504. Thus, it isbeneficial to keep frequency and delay search windows 501 and 502 assmall as possible, especially for indoor operation.

With continuing reference to FIG. 5 and renewed reference to FIG. 1,frequency window 501 is a function of Doppler uncertainty (due to therelative motion of GPS device 10 with respect to GPS satellite 11) aswell as frequency uncertainty due to imprecision of reference oscillator138 (shown in FIG. 4) in GPS device 10. Frequency offset 35A (shown inFIG. 3) provides an accurate estimate of offset from a referenceoscillator 138 (shown in FIG. 4) as frequency correction signal 31 isconventionally transmitted at a precise frequency. Thus, contribution ofreference oscillator 138 uncertainty to the frequency window 501 may besubstantially reduced or eliminated with frequency offset 35A. It shouldbe noted that adjustment of frequency window 501 may occur using asoftware algorithm in program memory 145 (shown in FIG. 4), with nospecial purpose circuit for steering voltage or controlling frequency ofreference oscillator 138. Alternatively, offsetting of frequency window501 may be achieved by altering frequency of reference oscillator 138with reference oscillator steering circuit 142 (shown in FIG. 4).

With renewed reference to FIG. 3, time synchronization signal 32 may beobtained by GPS mobile device 10 to determine time offset 36. The natureof time synchronization signal 32 varies depending on the cellularnetwork. In CDMA systems, time synchronization signal 32 can comprise asynchronization channel. The synchronization channel is a common channelthat is broadcast over a cell coverage area. The pilot channel andsynchronization channel of a particular cellular base station use anidentical PN sequence, such as a PN sequence of 2¹⁵ chips. Additionally,the synchronization channel is modulated with a particular Walsh code,allowing it to be separated from paging and traffic channels usingdifferent Walsh codes. The synchronization channel carries a timingmessage 33. Specifically, in CDMA, the synchronization channel carries amessage containing a pilot PN offset that identifies time of day offsetof such pilot channel.

With renewed reference to FIGS. 3 and 4, in a CDMA compatibleembodiment, GPS device 10 may first detect a pilot channel of a nearbybase station 13 with cellular acquisition front end 131, then proceed todecode a synchronization channel being broadcast by the same basestation 13. GPS device 10 achieves synchronization to such a pilotchannel at a particular timing offset of GPS device 10 local timekeepingcounter 139. Shortly thereafter, GPS device 10 receives time message 33containing a time of day offset of such a pilot channel. GPS device 10uses time message 33, along with the timing offset of local timekeepingcounter 139, to compute time offset 36. Since, in CDMA, base station 13time of day is synchronized to GPS time used by GPS satellites 11 (shownin FIG. 1), time offset 36 provides an absolute offset between localtimekeeping counter 139 and GPS time.

In GSM systems, time synchronization signal 32 is a synchronizationburst transmitted periodically by base station 13 on one of severalfrequency slots assigned to cellular base station 13. Timesynchronization signal 32 contains a unique header, such as a knownsequence of bits, that identifies a starting point of a burst. Inaddition, time synchronization signal 32 carries a timing message 33that comprises, among other elements, a GSM time stamp associated withsuch synchronization burst. In a GSM compatible embodiment, GPS device10 receives time synchronization signal 32, and uses header informationtherefrom to identify a starting point of a synchronization bursttherein relative to local timekeeping counter 139. GPS device 10 usesthis information, combined with timing message 33, to compute timeoffset 36. In this manner time offset 36 provides an offset betweenlocal timekeeping counter 139 and GSM timing of base station 13. In someGSM networks, GSM timing is not synchronized with GPS time. Therefore,time offset 36 does not provide an absolute time offset to GPS time.Time offset 36 may however be used beneficially as an indicator ofrelative time, as discussed below.

Time accuracy of time offset 36 will be dependent on the cellularnetwork implementation. In systems such as CDMA that incorporate GPStiming within the cellular network, there is a high degree of timingaccuracy. In other networks, for example GSM, relative timing of a timesynchronization burst may be good, but an unknown offset may exist toGPS time. Finally in some systems, a time indicator may be an absoluteindicator, but with limited accuracy, for example time coming from acomputer server 23 (shown in FIG. 2), in which time and date weremanually set.

Depending on accuracy, available time offset 36 may be employed forseveral purposes within GPS device 10. If timing offset 36 has precisionsubstantially better than one millisecond, precise time component 41 oftiming offset 36 may be incorporated into frequency and delay searchwindow 45. Specifically, with additional reference to FIG. 5, it is wellknown that, in the general case when precise timing is not available,delay window 502 spans an entire period of C/A code, nominally onemillisecond (C/A code conventionally refers to codes available forcivilian applications). This is because timing of locally generated C/Acode within GPS baseband processor 137 is arbitrary relative to GPSsignals 12 (shown in FIG. 1). However, if precise time component 41 isavailable, locally generated C/A code can be timed relative to GPSsignals 12 (shown in FIG. 1). Specifically, GPS device 10 usestimekeeping counter 139 that is common to a C/A code generator withinGPS baseband 137 and to cellular acquisition baseband 136. Thus, timeoffset 36, determined from a time synchronization signal 32 as describedabove, can be used in conjunction with local timekeeping counter 139 toprogram a starting point of locally generated code relative to GPStiming. In this manner, an uncertainty component of delay window 502caused by an unknown relative timing of locally generated code issubstantially reduced or eliminated. A remaining delay window 502component is delay uncertainty related to unknown pseudorange and anyerror in precise time component 41. As discussed below, a pseudorangemay be estimated from satellite trajectory models and an estimate ofposition, such as a position information 34 b. Thus, if precise timecomponent 41 is accurate to substantially less than one millisecond,delay window 502 may be reduced to substantially less than onemillisecond.

With continuing reference to FIGS. 3, 4 and 5, reducing frequency window501 as an aspect and delay window 502 as another aspect, substantiallyreduces the total number of search bins needed to overall cover twodimensional frequency and delay search window 45. As mentioned, thisenables GPS receiver 10, or more particularly GPS baseband processor137, to search more rapidly, and therefore reduce the time needed toobtain GPS satellite signals 12 (shown in FIG. 1). Furthermore, areduced search window provides GPS mobile device 10 an opportunity todwell longer at each search bin. Longer dwells provide signal-to-noiseratio enhancements that can enable weak signal reception indoors.

Optionally, time offset 36 is provided to coherent averaging 50.Coherent averaging 50 improves signal-to-noise ratio in each search binby averaging correlation results from several consecutive cycles of C/Acode. When coherent averaging, impact of 50 bps navigation data bits ona GPS signal is to be considered. Specifically, due to navigation databits, a GPS signal undergoes a potential 180 degree phase transitionevery 20 cycles of C/A code. For signal-to-noise ratio enhancement,coherent averaging is performed over twenty consecutive cycles of C/Acode comprising a single navigation data bit. Furthermore, to enhanceperformance this averaging process should be synchronous with navigationdata bit timing, otherwise changing data bits may partially defeat suchan averaging process. For this reason, it is desirable to achievesynchronization of coherent averaging 50 with navigation data bittiming. Navigation data bit timing is uniform for all satellites 11(shown in FIG. 1) and synchronized with GPS time.

Timing of a data bit arriving at GPS device 10 is a function of localtimekeeping counter 139 as well as pseudorange delay between GPS device10 and a satellite 11 (shown in FIG. 1). Precise time component 41establishes a relationship between local timekeeping counter 139 and GPStime. Thus, if pseudorange is estimated as described below, precise timecomponent 41 may be used in conjunction with local timekeeping counter139 to control start and stop times of coherent averaging 50 so as tomake a coherent averaging interval coincident with incoming navigationdata bits.

Time of day 42 is the absolute component of time offset 36, converted tounits of GPS time units. This conversion can take several forms, forexample, a conversion from a Julian data system, or some othertimekeeping standard employed by the cellular network. Time of day 42may be utilized within GPS receiver 10 even in an application whereprecision of time offset 36 is not better than one millisecond, namely,when precise time component 41 cannot be generated. In particular, timeof day 42 provides a reference time for ascertaining satellite positions43. Specifically, time of day 42 provides a reference time for satellitetrajectory model 39. As satellites 11 move rapidly through the sky, itis preferable that time of day 42 be accurate, within approximately atleast ten milliseconds, so that errors in prediction of satellitepositions will be on the order of meters or less. If, however, time ofday 42 does not provide this level of accuracy, error in time of day 42may be solved for as part of a navigation solution. In the lattersituation, accuracy of time of day 42 is unimportant, and a roughestimate of time is sufficient, such as the time provided by a server ora real time clock. An example of such a method is Time Free GPS, asdescribed in more detail in co-pending application entitled “METHOD ANDAPPARATUS FOR TIME-FREE PROCESSING OF GPS SIGNALS” to Frank vanDiggelen, application Ser. No. 09/715,860, filed Nov. 17, 2000, now U.S.Pat. No. 6,417,801.

If time offset 36 has an arbitrary relationship to GPS time, time of day42 will not be directly available. However, time offset 36 may bebeneficial as an indicator of relative time. For example, GPS mobiledevice 10 may determine an initial time of day by the conventionalmethod of decoding the time of week (TOW) portion of a navigation datastream. TOW can be used to determine a relationship between cellularnetwork time and GPS time. For example, time offset 36 may representoffset between GSM system time and GPS time of day. Once thisrelationship is established, it may remain constant for extended timeperiods as cellular base stations use precise oscillators to generatetheir timing signals. Thus, GPS mobile device 10 may use time offset 36to determine time of day 42 based on a previously determinedrelationship between a cellular network and GPS time. In this manner,GPS mobile device 10 may be able to obtain positions in indoor operatingenvironments, utilizing a time synchronization burst from timesynchronization signal 32 to ascertain time of day. Furthermore, GPSmobile device 10 can function without a battery powered real-time clockto maintain time.

Cellular base station 13 may provide a cell identification number 34A.The details of this message vary with cellular network. Cellidentification number 34A may be used to look up location of cellularbase station 13 in a lookup table 37 stored in memory of GPS mobiledevice 10. This will give an approximate or estimated position 38 of GPSmobile device 10, namely, GPS mobile device 10 will be within the sectorassociated with longitude and latitude of communication tower 13location. As sector sizes vary from rural, suburban and metropolitanarea networks, this position estimate 38 will vary accordingly dependingon location of cellular base station 13 within one of theabove-mentioned area networks. In those instances where cellular basestation 13 is configured to provide its cell location 35B, cell locationlookup table 37 may be avoided and an estimate of position 38 providedbased on cell location signal 34B.

Estimate of position 38 is provided for line of sight calculation 40.Specifically, estimate of position 38 is combined with satellitepositions, velocities and clock estimates 43 to determine expectedpseudoranges and pseudorange rates 44, and unit vectors 49 between GPSdevice 10 and each GPS satellite 11 (shown in FIG. 1). Line of sightcalculation 40, pseudorange and pseudorange rates 44, unit vectors 49,and delay and frequency measurements 47, are sufficient for position,velocity, and time computation 48. The details of such computations arewell known and will not be repeated here for purposes of clarity.

Pseudo range and pseudorange rates 44 are provided to frequency anddelay search window 45. In particular, pseudorange rate provides anestimate of Doppler shift between GPS mobile device 10 and each GPSsatellite (shown in FIG. 1), allowing frequency window 501 to bedetermined. Similarly, pseudorange provides an estimate of timing delaybetween GPS mobile device 10 and each GPS satellite 11 (shown in FIG.1), facilitating determination of delay window 502. As mentioned above,pseudorange and pseudorange rate 44 are components of frequency anddelay search window 45, more particularly frequency uncertainty inreference oscillator 138 and time uncertainty of locally generated C/Acode tied to time keeping counter 139, both of which may besubstantially reduced by means of cellular acquisition signals 102.

Integrated circuit 135 may comprise a time keeping counter 139 forproviding clock signals to baseband processors 136 and 137. A referenceoscillator 138 may be used to provide a determined frequency within atolerance to timekeeping counter 139. A general purpose processor, suchas a microprocessor, 141 is coupled to receive information fromacquisition signal baseband 136 to provide an output to GPS baseband137, as described with reference to FIG. 3. Microprocessor 141 iscoupled to memory 146, which may comprise partitioned memory orindividual memories 144 and 145. For individual memories, program memory145 is used to store programming, as described with reference to FIG. 3,for using one or more cellular acquisition signals to provideinformation regarding satellite range and range rate. Accordingly,program memory may be a programmable, non-volatile memory, such as anEPROM, E²PROM, flash memory, and the like. Data memory 144 may be usedto temporarily store data for microprocessor 141. Accordingly, datamemory 144 may be a programmable volatile memory, such as DRAM, SRAM,and the like. Optionally, a docking station, data modem and/or networkinterface 143 may be coupled to microprocessor 141 for receiving one ormore satellite trajectory models. Optionally, a digital-to-analog (D/A)converter 142 may be coupled to microprocessor 141 to receive a digitalsignal of a frequency and convert it to an analog signal of the samefrequency for providing a steering voltage to reference oscillator 138.

It should be appreciated that the incremental circuitry in the GPSdevice to receive and utilize cellular acquisition signals is minimal.In particular, the scope and cost of this circuitry is far less thanthat of a complete cell phone, which would include transmissioncircuitry, digital signal processing circuitry, voice processingcircuitry, a protocol stack processor, and many other components. Thus,it is anticipated that a GPS system in accordance with one or moreaspects of the present invention may be manufactured with less cost thanthat to produce conventional AGPS system.

While the embodiments described herein have provided details for GSM andCDMA systems, it should be apparent that the invention can be employedin all types of cellular networks including iDEN, TDMA, AMPS, GPRS,CDMA-2000 and other 2.5 networks, and W-CDMA and other 3G networks.Furthermore, the invention can accept multiple types of cellularacquisition signals in a single device. In particular cellularacquisition front end 131 and cellular acquisition baseband 136 may beconfigured to incorporate simultaneous or sequential processing ofsignals from multiple networks. This would further facilitate use of aGPS device 10 anywhere in the world, not just within a prescribedcoverage region, and accordingly it would be desirable to provide anability to receive and use a set of cellular network signals.

Though GPS satellites were described, it should be appreciated that oneor more aspects of the present invention may be used with pseudolites,ground based transmitters that broadcast a PN code similar to a GPSsignal. Accordingly, the term “satellites”, as used herein, is intendedto include pseudolites and equivalents thereof. Moreover, the term“satellite signals” or “GPS signals” is intended to includesatellite-like and GPS-like signals from pseudolites and equivalentsthereof. Furthermore, though a GPS system was described, it should beappreciated that one or more aspects of the present invention areequally applicable to similar satellite positioning systems, includingwithout limitation the Russian Glonass system.

While the foregoing is directed to the preferred embodiment of thepresent invention, other and further embodiments of the invention may bedevised without departing from the basic scope thereof, and the scopethereof is determined by the claims that follow.

1. A method for determining position of a Global Positioning System (GPS) handheld device in proximity to a base station of a wireless network, the method comprising: obtaining from the base station without having to have fee-based access to the wireless network at least one of location information and identification information; and determining, responsive to the at least one of location information and identification information, a position estimate of the GPS handheld device.
 2. The method of claim 1, further comprising: computing, responsive to the position estimate, any of a pseudorange and a pseudorange rate.
 3. The method of claim 2, further comprising: computing, responsive to the position estimate, a pseudorange; and determining, responsive to the pseudorange, a window of delay search for receiving GPS signals.
 4. The method of claim 1, further comprising: synchronizing, responsive to the pseudorange, coherent averaging to navigation data bits.
 5. The method of claim 1, further comprising: computing, responsive to the position estimate, a pseudorange rate; and determining, responsive to a pseudorange rate, a window of frequency search for receiving a GPS signal.
 6. The method of claim 1, wherein the location information comprises a location of the base station.
 7. The method of claim 1, wherein the identification information is a cell identifier of the base station.
 8. The method of claim 8, wherein determining the position estimate comprises: using the cell identifier to obtain a location of the base station from a lookup table having a relationship between the location and the cell identifier.
 9. A Global Positioning System (GPS) mobile device, comprising: at least one antenna; a first front end communicatively coupled to the at least one antenna, the first front end being adapted to receive an acquisition signal from a wireless network; a second front end communicatively coupled to the at least one antenna, the second front end being adapted to receive a satellite signal from at least one satellite of a GPS network; a baseband processor adapted to (i) receive from the first front end a first signal that is associated with the acquisition signal, and (ii) receive from the second front end a second signal that is associated with the satellite signal; a reference oscillator communicatively coupled to the baseband processor; a general-purpose processor communicatively coupled to the baseband processor; and memory communicatively coupled to the general-purpose processor.
 10. The device of claim 10, wherein the device does not comprise a transmitter for transmitting signals to the wireless network.
 11. The device of claim 10, wherein the device is not combined with a handset for communicating with the wireless network.
 12. The device of claim 10, wherein the first and second front ends are formed in a single integrated circuit.
 13. The device of claim 10, wherein the baseband processor comprises a first baseband processor for receiving the first signal that is associated with the acquisition signal, and a second baseband processor for receiving the second signal that is associated with the satellite signal, and wherein the first and second baseband processors are combined in a single integrated circuit.
 14. The device of claim 10, further comprising: a timekeeping counter communicatively coupled with the reference oscillator and the baseband processor. 